This invention relates to a demodulator for use in demodulating a modulated signal into a demodulated signal in a digital communication system. It is to be noted here that the modulated signal is subjected to quadrature amplitude modulation and that the demodulated signal is produced in the form of a pair of digital signals which are assigned to an in-phase and a quadrature phase channels, respectively.
In a digital communication system, a modulator and a demodulator are included in a transmitting end and a receiving end, respectively, which may be coupled to each other through a radio channel. The modulator carries out quadrature amplitude modulation to produce the modulated signal while the demodulator carries out coherent detection so as to improve a carrier-to-noise ratio.
On carrying out the coherent detection, a carrier wave should be reproduced in the demodulator. For this purpose, the demodulator is equipped with a phase-locked loop circuit. It is to be noted here that the phase-locked loop circuit is kept in a pull-in state specified by a pull-in phase when synchronization is established in the demodulator.
As known in the art, the pull-in state is ambiguous in the phase-locked loop circuit. Such ambiguity of the pull-in phase often gives rise to abnormal demodulation, such as cross demodulation, inverse demodulation, which is different from normal demodulation. In the interim, the cross demodulation is such that data signals to be assigned to the in-phase and the quadrature phase channels appear in the quadrature phase and the in-phase channels, respectively. On the other hand, the inverse demodulation is such that each polarity of the digital signals in the demodulated signal is inversed relative to a normal polarity thereof.
Anyway, the abnormal demodulation reproduces a wrong demodulated signal different from an original data signal and should therefore be restored to the normal demodulation.
Heretofore, various methods are proposed to produce a correct demodulated signal identical with the original data signal. For example, differential logic conversion of the digital signals is carried out in the transmitter and the receiver ends in U.S. Pat. No. 4,285,062 issued to Y. Yoshida et al. Alternatively, ambiguity of the pull-in phase is removed without differential logic conversion in Unexamined Japanese Patent Publication Syo No. 56-43855, namely, 43855/1981.
Anyway, the above-mentioned methods logically process the digital signals produced in the demodulator so as to reduce or remove the ambiguity of the pull-in phase.
It is a recent trend that levels or values to be conveyed by the modulated signal increase in number in the quadrature amplitude modulation. For example, the number of the values becomes equal to 256 or so.
Herein, it is assumed that the ambiguity in the pull-in phase is present in the phase-locked loop circuit of the receiving end and that the abnormal demodulation occurs in the demodulator. In this event, it should be noted that a waveform of the demodulated signal is distorted relative to an optimum waveform thereof due to a linear distortion occurring in baseband transmission facilities of the transmitting and the receiving ends, in particular, due to a linear distortion of a band restriction circuit responding to a digital multiplexed signal in the transmitting end.
As a rule, distortion of a demodulated analog signal is compensated by controlling any circuitry in the baseband transmission facilities or an equalizer. Such compensation is effective when the number of values or levels of the modulated signal is equal to or smaller than sixty-four.
However, the linear distortion becomes serious when the number of values is increased to 256. As a result, the above-mentioned compensation becomes ineffective and the abnormal demodulation becomes unavoidable even when the above-mentioned methods are adopted. This is because distortion of the demodulated analog signal can not be removed even when the digital signals are processed in the conventional methods mentioned above.
Therefore, the abnormal demodulation inevitably occurs on carrying out the above-mentioned compensation and makes a bit error rate of the digital signals large. This makes the normal demodulation difficult in the demodulator.